1. Field of the Invention
The subject matter of the present invention is a method of preparing polyvinylidene fluoride (PVDF) coatings on metal and inorganically oxidic surfaces by powder coating, the PVDF in powder form being fused onto the surface in the presence of an adhesivizing filler during or after application. The new coating powder that is used for the purpose is also subject matter of the present invention.
2. Discussion Of The Prior Art
The use of PVDF for the production of coatings by powder coating has been known for some years. The application methods include the fluidized bed coating method with its variants, the electrostatic powder coating method, powder coating with compressed-air, flame spraying, and the electrostatic fluid bed process. In all these processes, the maintenance of the strength of adhesion is of special importance, especially in the case of contact with aggressive chemicals at high temperatures.
Unless special measures are taken in the above-named coating processes, diffusion processes and shrinkage processes take place in the substrate-and-coating system when this system is exposed to elevated temperatures, especially in the presence of aggressive chemicals. These processes within a short time result in a reduction of the strength of adhesion, resulting in sagging and in separation of the coating from the substrate.
To avoid these difficulties it has been proposed to apply to the substrate, prior to the baking-on of the PVDF coating, an adhesivizing agent consisting of a mixture of epoxy resins and PVDF dispersions (cf. U.S. Pat. No. 3,111,426). These adhesivizing agents have the disadvantage that they are thermally sensitive and have to be applied to the substrate by a time-consuming process.
It is furthermore known, through German Offenlegungsschrift, to improve the adhesion of PVDF coatings applied to a substrate by powder coating, by applying to the substrate, prior to the coating procedure, an adhesivizing primer containing, in addition to various amounts of PVDF, at least one inorganic substance of high resistance to acids and alkalies. This primer is applied by powder coating, thereby avoiding by disadvantages of the method of U.S. Pat. No. 3,111,426. The inorganic substances used in this method, such as silicon dioxide (quartz sand), graphite or chromium oxide, for example, give the surface of the primer a gritty structure which cannot be leveled out by a PVDF cover coat applied over it. The result is reduced resistance to wear in the coatings thus obtained.
Another disdvantage of these coatings lies in their insufficient porosity. Metallic objects which are covered with such coatings are therefore not sufficiently protected against corrosion, since in the course of time rusting and more or less severe corrosion occurs at the insufficiently sealed areas.
The problem therefore existed in providing PVDF coatings which can be applied to the substrate by powder coating, which will have better adhesion to the substrate, and which will have a very smooth surface. Furthermore, the coating thus obtained is to be non-porous.